Static charging of plastics is a problem when used in hospitals because of the presence of explosive gases such as ethers. Therefore, anti-static agents have either been incorporated into the plastic or sprayed on the external surface. In chemical protective garments, tear resistance and abrasion resistance are important so that incorporating the anti-static agents into the plastic is generally not employed because they weaken the plastic. Tear and abrasion resistance is important in military and firefighting protective garments so that a weakening of the plastic film is avoided. External and anti-static agents can be abraded off or inactivated by external chemicals reactions and can be generally less effective because they are not concentrated at the surface of the film or fabric where they are most needed.
It is generally assumed that interfacially active molecules of some anti static agents function by accumulating on the surface and are oriented with the hydrophobic part containing a paraffin chain extending into the plastic and a hydrophilic part pointing outwards where it is able to adsorb water on the surface. In consequence of their interfacially active character, antistatic agents decrease the phase boundary angle between water and plastic, thus allowing water to be uniformly distributed on the surface. A water film, whose thickness depends on the atmospheric humidity, forms on the plastics' surface, thus increasing the conductivity by means of an ion conduction process and dissipating static charge build-up. This also explains why the surface conductivity and hence the antistatic action progressively decrease with decreasing atmospheric humidity.
More recently it has been postulated that charge exchange, in addition to the ion conduction mechanism, is effected through a constant exchange of water between the surface and the environment. This would be in accord with the mechanism of air ionization. In contrast to this, however, the antistatic agent on the surface acts as a contact point for the charge exchange.
In addition, charge transfer can be accomplished by a proton shift. Antistatic agents bearing —CO2H, OH or NH2, —SO3H groups are able to associate in chain form via hydrogen bonding, and display antistatic activity even at low atmospheric humidity, unlike compounds which are able to form only intramolecular hydrogen bonds.
Many antistatic agents also show hygroscopic properties, thereby intensifying the attraction of water to the surface. At constant atmospheric humidity a hygroscopic compound combines more water at the surface and so increases the antistatic effectiveness.
Application of antistatic agents in the normal concentration range brings about a decrease in the surface resistivity from 1014–1016 to approximately 108–1010Ω. A further decrease is attainable only by increasing the surface conductivity.
Many antistatic agents in use today are classified as cationic, anionic, nonionic, and amphoteric compounds. The molecules have a hydrophilic part and a hydrophobic component. The hydrophobic part confers a certain compatability with the particular polymer and is responsible for anchoring the molecule on the surface while the hydrophilic part takes care of the binding and exchange of water on the surface.
A number of external agents are applied to the external surface of molded articles from aqueous or alcoholic solutions. Hygroscopic agents such as glycerin, polyols, and polyglycols, quaternary ammonium salts, sodium alkyl sulphonates and fatty acid monoglycerides appear to be the preferred choices for external antistatic activity.
The problem with external antistatic agents is their decreased activity in low humidity areas and under arid conditions. Moreover, external antistatic agents can be washed off, rubbed off, or inactivated by chemical agents.